Current source with positive feedback current repeater

ABSTRACT

A two-terminal current-limiting or current source arrangement is coupled across a source of voltage which is subject to variations. The limiter includes a current determining resistor connected in the base-emitter circuit of a first transistor. A second transistor provides negative feedback between collector and base of the first transistor and also provides a collectoremitter circuit in series with the resistor. A current repeater is connected between the collectors of the first and second transistors and produces positive feedback to the base of the second transistor. The repeater includes a transistor, the collector-emitter circuit of which is connected to the collector of the first transistor. The voltage source is of sufficient magnitude to bias the repeater transistor and second transistor to nonsaturated conduction.

United States Patent [72] Inventor Ronald Bruce Goyer Hollywood, Calif. [2]] Appl. No. 105,520

[22] Filed Jan. 11, 1971 [45] Patented Dec. 21, 1971 [73] Assignee RCA Corporation [54] CURRENT SOURCE WITH POSITIVE FEEDBACK CURRENT REPEATER 10 Claims, 2 Drawing Figs.

[52] US. Cl. 323/4, 307/297 [51] Int. Cl 005i 1/56 [50] Field of Search 307/296. 297; 323/1. 4, 9

[56] Relerences Cited 7 UNITED STATES PATENTS 3,246,233 4/1966 Herz 3,573,504 4/1971 Breuer .L 3,577,167 5/1971 Avins Primary ExaminerA. D. Pellinen Attorney-Eugene M. Whitacre ABSTRACT: A two-terminal current-limiting or current source arrangement is coupled across a source of voltage which is subject to variations. The limiter includes a current determining resistor connected in the base-emitter circuit of a first transistor. A second transistor provides negative feedback between collector and base of the first transistor and also provides a collector-emitter circuit in series with the resistor. A current repeater is connected between the collectors of the first and second transistors and produces positive feedback to the base of the second transistor. The repeater includes a transistor, the collector-emitter circuit of which is connected to the collector of the first transistor. The voltage source is of sufficient magnitude to bias the repeater transistor and second transistor to nonsaturated conduction.

PATENTEU 00221 l9?! 3,529,592

INVENTOR.

Fan/4m 5 60 x54 KW CURRENT SOURCE WITH POSITIVE FEEDBACK CURRENT REPEATER This invention relates to electrical circuit arrangements for providing a relatively constant, predetermined current from a source of potential which may vary over a wide range of values.

Such circuit arrangements are particularly adapted for construction in monolithic integrated circuit form and will therefore be described in terms of that environment.

The circuit arrangements may be used to provide predetermined current levels such as are employed for biasing semiconductor amplifiers. Alternatively, these circuit arrangements may be used as two-terminal current limiters to isolate a utilization device from variations in an associated source of operating potential.

In a preferred embodiment of the invention, a currentdeterrnining resistor is coupled between the base and emitter electrodes of a transistor arranged in a common emitter configuration. The emitter of the transistor is coupled to a reference terminal. Current is supplied to the resistor by the emitter-collector path of a second transistor, the base-emitter circuit of which is connected to provide negative feedback between the collector and base of the first transistor. A diodeconnected transistor and a fourth transistor having proportionally related (e.g., equal) conduction characteristics are coupled between an operating potential supply and the collectors of the second and first transistors, respectively, so as to provide positive feedback between the collector and base of the second transistor. The arrangement limits the current between the potential supply and reference terminal to a substantially constant level for a wide range of potentials above a predetermined threshold. The current level is selected by choosing the value of the included resistor.

The novel features that are considered characteristic of this invention are set forth in the appended claims.

The invention itself, however, both as to its organization and method of operation, as well as additional objects, will best be understood from the following description when read in connection with the accompanying drawing, in which:

FIG. 1 illustrates in schematic circuit form a current-limiting arrangement constructed in accordance with the present invention; and

FIG. 2 illustrates in schematic circuit diagram form a modified version of the embodiment shown in FIG. 1 which may be employed as a current limiter or as a current source.

Referring to FIG. 1, all of the illustrated components are suitable for construction on a single, monolithic integrated circuit chip. I

A source of operating potential (8+) is coupled between terminals 10 and 12, the latter being coupled to the reference or ground terminal of the B+ source. A biasing arrangement, such as may be employed on an integrated circuit chip for biasing a cascade-connected differential amplifier (not shown), is connected between terminals 10 and 12. The biasing arrangement includes the series combination of diode-connected transistors 14 and 16 (shown as NPN types), a voltage dropping resistor 18, a diode-connected transistor 20 (shown as a PNP type), a two-terminal current-limiting circuit 22 and a diode-connected transistor 34 (shown as an NPN type). Diode-connected transistors l4, 16, 20 and 34 are shown for the purpose of illustrating one type of arrangement with which current limiter 22 may be employed. It should be recognized that various arrangements of components may be employed in such a bias circuit. For example, depending upon the desired voltage-current relationship, each illustrated diode-connected NPN transistor may actually be a combination of a PNP and an NPN diode-connected transistor or a Darlington arrangement of diode-connected transistors to provide an offset voltage greater than V (i.e., the voltage across a forward conducting base-emitter junction which is of the order of 0.6 volt for a silicon transistor of the type employed in integrated circuits). The current limiter 22 is constructed in accordance with the present invention and includes two main current paths. The first current path includes a resistor 24, the

emitter-collector path of an NPN-transistor 26 and a diodeconnected PNP-transistor 28. The second current path includes the emitter-collector circuit of an NPN-transistor 30 and the collector-emitter circuit of a PNP-transistor 32.

Resistor 24 is connected between the base and emitter of transistor 30. The base of transistor 26 is connected to the collector of transistor 30 so as to provide negative (degenerative) feedback between collector and base of transistor 30. Diodeconnected transistor 28 and transistor 32 have their input (base-emitter) circuits connected in parallel and are arranged to exhibit proportionally related (e.g., equal) conduction characteristics. The combination of transistors 28 and 32 therefore functions as a current repeater which, for purposes of explanation, will be assumed to exhibit substantially unity gain. Transistors 26, 28 and 32 are arranged in a regenerative (positive) feedback configuration.

In the operation of the circuit of FIG. 1, current limiter circuit 22 serves to maintain a predetermined current through the bias elements 14, 16, 18, 20 and 34 despite variations in the 8+ supply voltage. The voltage offsets (drops) across each of the bias elements therefore are also stabilized despite variations in B+ voltage. The manner in which current limiter circuit 22 accomplishes this purpose will now be explained.

in the case where transistors 28 and 32 are fabricated as substantially identical devices in close proximity (i.e., same thermal environment) on a single integrated circuit chip, substantially equal collector currents will be produced in devices 28 and 32 since their input circuits are connected in parallel. Assuming, for purposes of explanation, that the base currents of the devices are small compared to corresponding collector currents, in the FIG. 1 arrangement, substantially one-half of the current supplied to the joined emitters of transistors 28 and 32 will flow through the first current path including the emitter-collector of transistor 28, the collector-emitter of transistor 26 and resistor 24. The other half of the applied current will flow through the second current path including the emitter-collector of transistor 32 and the collector-emitter of transistor 30.

It can be seen that the collector of transistor 30 is maintained at 2V, above its emitter by virtue of its own baseemitter ofiset and the connection of the baseemitter of transistor 26 from collector to base of transistor 30. [t can also be seen that the collector of transistor 26 (and the base of transistor 32) is one V below the joined emitters of transistors 28 and 32. Therefore, when the total voltage across current limiter circuit 22 is less than approximately 3V (e.g., 1.8 volts), the collector-base junctions of each of transistors 26 and 32 are biased such that transistors 26 and 32 are in saturation. in that case, the limiter circuit 22 exhibits the characteristic of a resistor equal substantially to one-half the value of resistor 24 and limiting action does not occur.

However, when the voltage applied across limiter circuit 22 exceeds approximately 3V,, transistors 26 and 32 operate as nonsaturated devices and provide relatively high impedances in each current path which vary with applied voltage so as to maintain a substantially constant current through limiter 22. The magnitude of the constant current is substantially equal to the sum of the current in resistor 24 plus the collector-emitter current of transistor 30. Because of the above-described characteristics of transistors 28 and 32, these two currents are substantially equal. The current in resistor 24 is limited to that value required to produce a voltage across resistor 24 equal to the V associated with transistor 30 when it passes a like current. This current value may be determined, for example, utilizing the well-known diode equation:

I, emitter current of transistor 30 in amperes I, saturation current of transistor 30 (typically 0.2Xl0" amperes for integrated transistors) e natural logarithm base q= charge on an electron in coulombs V= base-emitter voltage of transistor 30 in volts k Boltzmans constant T= operating temperature in' degrees Kelvin As stated above, the base-emitter voltage of transistor 30 is equal to the product of the value of resistor 24 and the current through transistor 30, the latter being substantially equal to I for the specified conditions. Substituting I,R for the voltage V in the above expression yields:

0.6 (volt) limit (amps) R 4 (ohms) A more accurate selection of resistorvalue may be made by determining the base-emitter offset voltage of transistor 30 for a particular current from characteristic curves of the device. These current and voltage values may then be substituted in the above equation.

In a typical circuit, with the value of resistor 24 equal to 15,000 ohms, limiter circuit 22 provides a substantially constant 80 microamperes of current despite the applied voltage varying to several times the threshold value required for limiting to occur. Depending upon the actual design selected for transistors 26 and 32 as related to their collector to base voltage breakdown characteristics, an applied voltage ,(B+) of up to 30 volts or even more can be accommodated by limiter circuit 22 while still providing a substantially constant 80 microamperes of current.

Various modifications may be made in the limiter circuit 22 illustrated in FIG. 1. For example, the current relationship between transistors 28 and 32 may be other than unity. That is, the geometries of transistors 28 and 32 may be other than identical. If, for example, the emitter-base junction of transistor 28 is fabricated larger than that of transistor 32, the current in the first path including resistor 24 will be proportionally greater than that in the second path including transistor 30. In that case, the value of resistor 24 would be reduced accordingly to obtain the desired limit current. Alternatively, transistor 32 may be fabricated with a larger emitterbase junction than transistor 28 and an appropriate change would be required in the value of resistor 24. Equivalent operation may also be obtained by coupling like transistors in parallel with transistor 32 (or by coupling like diode-connected transistors in parallel with transistor 28 to obtain the first-mentioned operation).

Additional components may also be added to the simple configuration illustrated (e.g., transistor 26 may be replaced by a more complex amplifier). lt should also be noted that a negative voltage supply may be coupled to terminal 12 and, in that case, terminal may be connected to a reference voltage such as ground.

FIG. 2 illustrates a limiter arrangement 22' similar to that shown in FIG. 1 wherein each of the transistors 26', 28', 30 and 32 is of opposite conductivity as compared to the cor respondingly numbered transistors of FIG. 1. The circuit shown in FIG. 2 includes a current repeater 28', 32 employing NPN transistors. Typically, the current gain of such NPN devices is greater than that of PNP devices constructed in integrated circuit form. The currents through transistor 30' (emitter-collector) and resistor 24' therefore more closely approximate the proportionality determined by the ratio of the base-emitter junction geometries (i.e., areas) of transistors 28' and 32'. t

Since the emitter of transistor 30' is connected to ground, it is also convenient to duplicate the current flowing in transistor 30' by coupling the base and emitter of a further transistor (not shown) directly across the corresponding terminals of transistor 30'. Thus, a current source stabilized against supply voltage variations may be provided.

What is claimed is:

l. A current-limiting circuit comprising a first semiconductor device having base, emitter and collector electrodes,

a resistor coupled between said base and emitter electrodes,

means including a second semiconductor device having a collector-emitter circuit coupled to said resistor for supplying current to said resistor and having a base-emitter circuit coupled between said collector and base electrodes of said first device,

means, including third and fourth semiconductor devices having proportionally related conduction characteristics, for providing positive feedback between said collector and base electrodes of said second device, and

means coupled between said emitter of said first device and said positive feedback means for supplying energized potential to said devices.

2. A current-limiting circuit according to claim 1 wherein said third and fourth devices comprise input circuits coupled in parallel, and said third and fourth devices further comprise output circuits coupled respectively, to the collector-emitter circuits of said second and first devices.

3. A current-limiting circuit according to claim 2 wherein each of said semiconductor devices comprises a transistor having base, emitter and collector electrodes, the base and collector electrodes of said third transistor being directly connected together.

4. A current-limiting arrangement according to claim 3 wherein said energizing potential is of sufficient magnitude to bias said second and fourth transistors to nonsaturated condition.

5. A current-limiting circuit according to claim 4 wherein said first and second transistors are of a first conductivity type and said third and fourth transistors are of a second conductivity type.

6. A current-limiting circuit according to claim 5 wherein said third and fourth transistors have substantially identical conduction characteristics, and

said resistor is selected with a value equal to twice the baseemitter offset voltage of said first transistor divided by a predetermined limit current value.

7. An electrical circuit for providing a predetermined current comprising a first transistor having base, emitter and collector electrodes,

a resistor connected between said base and emitter electrodes,

a second transistor having base, emitter and collector electrodes, said emitter of said second transistor being connected to said base of said first transistor, said base of said second transistor being coupled to said collector of said first transistor,

a diode-connected transistor and a fourth transistor, each having base, emitter and collector electrodes, and exhibiting proportionally related conduction characteristics, the collector electrode of said diode-connected transistor being connected to the collector of said second transistor and to the base electrodes of said diode-connected and fourth transistors, the collector electrode of said fourth transistor being connected to the collector electrode of said first transistor, and

a source of voltage coupled from said emitter electrodes of said diode-connected and fourth transistors to the emitter electrode of said first transistor, said voltage being of sufficient amplitude to maintain said second and fourth transistors in nonsaturated conjunction.

8. An electrical circuit according to claim 7 wherein transistor.

10. An electrical circuit according to claim 9 wherein said diode-connected and fourth transistors exhibit substantially identical conduction characteristics, and

said resistor is selected equal to twice the base-emitter offset voltage of said first transistor divided by a predetermined limit current.

l It i l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 629,692 Dat d December 21, 1971 Inventor( g) Ronald Bruce Goyer It is certified that error appears in the above-identified patent and that said Letters Patent: are hereby corrected as shown below:

"Col n- 1, 1111954,: "cascade" should read cascode -b Col. 3, line 3', "I should read I line 12,

"ELI-H should read E q q I Col. 4, line 74 "conjunction" should read conduction Signed and sealed this 28th day of November 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) uscoMm-oc 00376-P59 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 629,692 Dat d December 21, 1971 Inventor( g) Ronald Bruce Goyer It is certified that error appears in the above-identified patent and that said Letters Patent: are hereby corrected as shown below:

"Col n- 1, 1111954,: "cascade" should read cascode -b Col. 3, line 3', "I should read I line 12,

"ELI-H should read E q q I Col. 4, line 74 "conjunction" should read conduction Signed and sealed this 28th day of November 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM PO-IOSO (10-69) uscoMm-oc 00376-P59 UNITED STATES PATENT ormcn CERTIFICATE Q CORREQHQN Patent No. 3,629,692 Dated December 21, -l97l Inventor(g) Ronald Bruce Goyer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1, line 54, "cascade" should read cascode Col. 3, line 3, "I should read I line 12, "55" should read 2 Col. 4, line 74, "conjunction" should read conduction Signed and sealed this 28th day of November 1972.

(BEAM Attest:

EDWARD M@FLETCHER,JRo ROBERT GOTTSCHALK ttesting Officer Commissioner of Patents ORM PO-IOSO (O-69) USCOMM-DC 00376-P69 

1. A current-limiting circuit comprising a first semiconductor device having base, emitter and collector electrodes, a resistor coupled between said base and emitter electrodes, means including a second semiconductor device having a collector-emitter circuit coupled to said resistor for supplying current to said resistor and having a base-emitter circuit coupled between said collector and base electrodes of said first device, means, including third and fourth semiconductor devices having proportionally related conduction characteristics, for providing positive feedback between said collector and base electrodes of said second device, and means coupled between said emitter of said first device and said positive feedback means for supplying energized potential to said devices.
 2. A current-limiting circuit according to claim 1 wherein said third and fourth devices comprise input circuits coupled in parallel, and said third and fourth devices further comprise output circuits coupled respectively, to the collector-emitter circuits of said second and first devices.
 3. A current-limiting circuit according to claim 2 wherein each of said semiconductor devices comprises a transistor having base, emitter and collector electrodes, the base and collector electrodes of said third transistor being directly connected together.
 4. A current-limiting arrangement according to claim 3 wherein said energizing potential is of sufficient magnitude to bias said second and fourth transistors to nonsaturated condition.
 5. A current-limiting circuit according to claim 4 wherein said first and second transiStors are of a first conductivity type and said third and fourth transistors are of a second conductivity type.
 6. A current-limiting circuit according to claim 5 wherein said third and fourth transistors have substantially identical conduction characteristics, and said resistor is selected with a value equal to twice the base-emitter offset voltage of said first transistor divided by a predetermined limit current value.
 7. An electrical circuit for providing a predetermined current comprising a first transistor having base, emitter and collector electrodes, a resistor connected between said base and emitter electrodes, a second transistor having base, emitter and collector electrodes, said emitter of said second transistor being connected to said base of said first transistor, said base of said second transistor being coupled to said collector of said first transistor, a diode-connected transistor and a fourth transistor, each having base, emitter and collector electrodes, and exhibiting proportionally related conduction characteristics, the collector electrode of said diode-connected transistor being connected to the collector of said second transistor and to the base electrodes of said diode-connected and fourth transistors, the collector electrode of said fourth transistor being connected to the collector electrode of said first transistor, and a source of voltage coupled from said emitter electrodes of said diode-connected and fourth transistors to the emitter electrode of said first transistor, said voltage being of sufficient amplitude to maintain said second and fourth transistors in nonsaturated conjunction.
 8. An electrical circuit according to claim 7 wherein said first and second transistors are of a first conductivity type, and said diode-connected and fourth transistors are of a second conductivity type.
 9. An electrical circuit according to claim 8 wherein the magnitude of said voltage is greater than the sum of the collector to emitter voltage of said first transistor plus the base-emitter offset voltage of said diode-connected transistor.
 10. An electrical circuit according to claim 9 wherein said diode-connected and fourth transistors exhibit substantially identical conduction characteristics, and said resistor is selected equal to twice the base-emitter offset voltage of said first transistor divided by a predetermined limit current. 